Microseismic monitoring, that is, acquisition and processing of data connected with microseismic events, has been used for some time to infer the locations of faults and joints in the subsurface of the earth. Earthquake monitoring is typically performed by placing arrays of instruments at or near the surface of the earth. These instruments sense the motion of the earth caused by the propagation of seismic energy from the microseismic events, and convert this motion into corresponding data related to the events. These data may then be used to locate the origin of the seismic energy, that is, the microseismic event, in the subsurface of the earth.
More recently, microseismic techniques have been used to monitor fractures occurring in the subsurface of the earth as a result of hydraulic fracturing. Hydraulic fracturing is the process of creating or enhancing fractures in rock formations by pumping fluid at high pressure into a well bore, causing the surrounding geologic layers to fracture. Fracturing causes seismic events that emit energy in the form of seismic waves. The magnitude of these events is typically less than zero on the Richter scale. This seismic energy can be detected and mapped to show the location and the extent of the fractures created or enhanced by the hydraulic fracturing operation. Microseismic monitoring is typically performed by placing arrays of instruments in wells or boreholes, or near or at the surface of the earth, in the vicinity of the hydraulic fracturing operation.
The purpose of this microseismic monitoring is to determine if the hydraulic fracturing has had the intended effects within the hydrocarbon-bearing rock formation, and whether there are any unintended effects, such as opening fractures into shallower layers or groundwater aquifers. Microseismic monitoring is often performed in real time during the hydraulic fracturing operation, in which case the fracturing operation can be modified or stopped if unintended fracturing events are evident.
In microseismic surveys, the source of the energy is a fracture in the subsurface of the earth, which produces a very low level of seismic energy, hence the amount of energy that reaches the surface and is detected by the geophones is extremely small. Therefore microseismic monitoring is limited by noise contamination. Noise contamination may include surface waves, refracted waves, and reflected waves, that is, seismic energy from surface noise sources that is transmitted into the earth and reflected back from within the subsurface. Reflected noise waves are particularly problematic because the angle of incidence of their arrival at the surface is very close to the angle of incidence of energy from microseismic events, making reflected noise essentially impossible to separate and remove by conventional methods. Noise contamination masks microseismic signals and can lead to the false identification of noise as microseismic events.
What is desired are improved techniques wherein surface microseismic data are acquired and processed in such a manner that most of the noise is eliminated, thereby allowing detection and location of more and smaller microseismic events. The resulting improvement in the signal to noise ratio of the microseismic data allows for better event locations that in turn may more accurately represent the effects of hydraulic fracturing, and avoid false events that may misrepresent the effects of hydraulic fracturing.